Frequency measuring circuit



1949 J. A. RAJCHMAN arm.- 2,492,353

I FREQUENCY MEASURING CIRCUIT I Original Filed April 1, 1942 2 Sheets-Sheet 1 .mano k r 1 mil N 1949 J. A. RAJCHMAN HAL. 3 3

FREQUENCY MEASURING CIRCUIT I Original Filed April 1, 1942 2 Shets-Sheei 2 nnnnnnn rs Jan H. Ra 'ahman 8: Edwin H. oldberg Ctttomeg Patented Dec. 27, 1949 FREQUENCY BH'JASURING CIRCUIT Jan A. Rajchman, Princeton, and Edwin A. Goldberg, Princeton Junction, N. J., assignors to Radio Corporation of America, a corporation of Delaware Original application April 1, 1942, Serial No. 437,260. Divided and this application April 29, 1948, Serial No. 23,908

3 Claims. 1

This invention relates generally to frequency measuring circuits and particularly to a frequency measuring circuit in which a current is derived which is substantially proportional in amplitude to the average frequency of a signal applied to the input of the circuit, the present application being a division of our copending application Serial No. 437,260, filed April 1, 1942, now Patent No. 2,467,777.

The invention utilizes a unique arrangement of thermionic tube circuits including a high pass filter, one or more saturation amplifiers, a difierentiating circuit, a peak amplifier, and a novel trigger circuit, as well as means for damping the diiierentiating circuit and the trigger circuit.

Among the objects of the invention are to provide means for deriving a current or voltage proportional in amplitude to the average frequency of a signal to be measured. Another object of the invention is to provide means for measuring the average frequency of an applied signal by pulsing said signal, by applying said pulses to a trigger circuit and deriving a current from said trigger circuit which is proportional in amplitude to the average frequency of the applied signal. Still another object is to provide a novel trigger circuit from which may be derived a current proportional in amplitude to the average frequency of the signal applied to key the trigger circuit.

.The invention will be described hereinafter by reference to the drawings, of which Fig. 1'is a schematic circuit diagram of the invention; and Fig. 2 is a series of graphs indicating theoperation of the various circuit components.

Referring to Fig. 1, signals from any suitable source, which may include a plurality of frequency components, are applied to the input terminals ID of a filter circuit -9 which is designed to pass the frequency band to be measured. The output of the filter 9 is applied to the grid circuit of a first thermionic tube I. The grid bias is adjusted to limit the amplitude of the signals to be measured in order to eliminate, as much as possible, response to extraneous signals such as harmonics higher than the fundamental frequency. The first tube l is operated at the sat uration portion of its static characteristic in order to derive an output signal which is substantially of square wave form. The signal is further amplified by a second thermionic tube 2 which is also operated at the saturation point of itsstatic characteristic in order to further improve the square wave form of the signal. The signal of substantially square Wave form is next applied to the input circuit of a third thermionic tube 3.

The anode circuit of the third tube 3 includes a two-position switch 20 which is connected in one position to one terminal of a resistor I9 and in another position to one terminal of an inductor I I. The movable arm of the switch 20 is connected to the cathode of a first diode 4 and to one terminal of the capacitor 2|. The remaining terminals of the resistor l9, inductor H and the anode of the diode 4 are all connected through an anode resistor 24 to the source of high potential for the anode of the third tube 3. The remaining terminal of the capacitor 2| is connected to the control electrode of a peak amplifier 5, which is biased to amplify only the voltage peaks of the applied signal. The cathode circuit of the peak amplifier 5 includes a cathode resistor 22. Voltage across this resistor is applied to the cathode circuit of a first trigger tube 7. The control electrode of the first trigger tube 1 is connected to the anode of a second diode 6, to one terminal of the grid resistor l5, and to one terminal of the capacitor l3. The cathode of the second diode 6 and the remaining terminal of the resistor I5 are connected to ground. The remaining terminal of capacitor I3 is connected to the anode of a second trigger tube 8 and to one terminal of a resistance network 23. The remaining input terminal of the resistance network 23 is connected to a source of anode potential for the second trigger tube 8. The anode of the first trigger tube 1 is connected to the control electrode of the second trigger tube 8 and to one terminal of a coupling resistor M. The remaining terminal of the resistor I4 is connected through the resistor 25 to a source of anode potential for the first trigger tube 1.

The operation of the circuit is as follows: The desired frequency component of the signal to be measured is derived from the filter 9 and applied to the control electrode of the first tube l which provides high amplification and, because of its saturation characteristics, clips the peaks of the signal wave. The signal is further amplified and clipped by a similar action in the second tube 2 and applied as a signal of substantially square wave form to the input of the third tube 3. When the switch 2!) is connected to the inductor l I, the third tube 3 is operated to shock-excite the tuned circuit comprising the natural resonant characteristics of the inductor H, to derive a series of pulses of decreasing amplitude from each square wave pulse applied to the circuit. The first diode 4 provides considerable damping of the pulses of decreasing amplitude to eliminate substantially all of the pulse signal except the first positive alternation. The resistance capacity network I9'-2l acts as a differentiating circuit. In this network the voltage across the resistor l9 will be substantially proportional to the rate of a e all the square; wave signal applied tothe. network and will therefore include. only a sharp positive and negative pulse for each cycle of the square Wave signal. When the switch 20 is cone nected to the resistor IS, the damping diode 4 may be omitted, since it will have little effect on the circuit operation.

The differentiating network does not. substantlally affect the first positive alternation derived from the coil H because the oscillatory voltage generated across the coil is of high frequency. A square wave voltage signal is not applied to the network when the coil II is in the circuit. In other words, two alternate methods for generating pulses are shown. With the coil iii in the circuit, the pulses are generated by shock exciting the coil and its. effective shunting and distributed capacity into high frequency oscillation by means of av steeplyrising plate current wave, and dampi-ng out all oscillations after the first positive one by means of a damping diode. With the coil not: the circuit, the derivative of a square wave of voltage appearing across the resistor I9 is taken by the differentiating network or the ca.- pacitor 2| and the resistor i=9 Signals. derived from the circuit with eitherposition of the switch 2d are then applied as pulses: to the control electrode of the peak ampl'ifiei. r If desired, either the inductor It or the resistor l9, and the switch may be omitted.

The peak. amplifier is biased to amplify only the positive: peak portion of the pulse applied to the control electrode. Sharply peaked voltages from the cathode circuit of the peak amplifier 5 are applied to the input circuit of the first trigger tube; 1:.

The. operation of the. trigger circuit is as follows: The first trigger tube T is biased so that it is normally conducting while the second trig gertube 8 is. biased so that it is normally nonconducting. When a positive pulse from thepeak amplifier 5 is applied to the cathode of the first trigger tube l, the first trigger tube 1 is biased to cut-off and the second trigger tube 8; is made to conduct. This condition continues after the exciting pulse has passed, and; until the grid. of the first trigger tube 1, which has been driven to cut oil: by the charge on the capacitor I3, becomes sufficientlypositive for the first trigger tube 1 to again become conducting and the second trigger tube 8 nonrconductingl For a single exciting pulse, the time during which the second trigger tube 8 will become conducting depends upon the capacitance of the capacitor t3, the grid capacitance of the first trigger tube 'l. the resistance of the resistors i4 and 15. the cutofi'voltage of the first trigger tube I, and the rate of change of the maximum voltage on the anode of the. second trigger tube 8. when the tube is suddenly made to conduct. Since all of these constants can be calculated and fixed, the circuit can be adjusted to any desired time constant.

The limit frequency of the circuit is dependent on the time required for the trigger tubes to. return to their normal bias condition after actuation by an exciting pulse. This time interval may be greatly reduced by the use of the secand diode 6 which has a damping action on the grid circuit of the first trigger tube f by providing substantial attenuation in the circuit when the grid of the first trigger tube 1 is at positive Gil.

potential. The action of the diode 6 also tends to make the duration of the current pulse in the anode circuit of the second trigger tube 8 more uniform. The amplitude of this pulse may be maintained at, a substantially constant level by proper voltage regulation of the potentials applied to the trigger tube circuits. The current derived from the output terminals l2 of the resistance. network 23 will be a fairly accurate indication of the average rate of occurrence of the exciting; pulses applied to the cathode of the first trigger tube E.

Fig; 2a of the drawing shows a typical sine wave signal applied to the input circuit of the first saturation amplifier tube I. Fig. 2b shows a signal of substantially square wave form derived item the anode, circuit of the second tube 2 and applied to the input circuit of the tube 3. Fig. 2c shows the wave form comprising pulses of diminishing amplitude derived from the tuned circuit. it when the. switch 21k is. connected to the inductor tl. The right-hand portion shows; the wave form derived from the differentiating networkv lit"; 2|! when the switch 20 is. connected to the resistor I9. Fig. 2d shows. the damping. of the pulse currentv by the first. diode 4. The portion of the graph above the. dash line B indicates the positive portion ofthe. pulse current which ac.-v tuates the peak amplifier 5.. Fig. 2e showsv the positive, pulse derived from across the resistor 22 in the cathode circuit of the peak amplifier 5. Fig. 2a shows the potential variations on the grid of the first. trigger tube I caused by the application to the tube cathode of the pulse shown in Fig. 2c. Fig. shows the corresponding potential variations in the anode circuit of the second trigger tube 8 which are applied to the resistance network 23. The dashed lines in Fig. 2 indicate the dampingaction of the second diode Ii and clearly show the action of this tube in decreasing the time required for the trigger tubes 1- and 8- to return to their normal bias condition.

It should be understood that the filter 9, tubes I, Z, 3, 4 and 5, or any of them,'may be omitted if' the signal to be measured has suitable characteristics for the actuation of the trigger circuit comprising the tubes- 6, l, and ii. It should also be understood that the second diode 6 may be omitted if the operating frequency of the circuit is sufficiently low to permit the trigger tubes '1 ancl'8 to return to normal bias condition without the damping action of the diode 6.

We claim as our invention l. A device for shaping the waveform of applied signal voltages including, in combination, a saturation amplifier, means for applying said signals to said amplifier to derive a current of substantially square wave form, a tuned circuit, means for shock exciting said circuit by said square wave current to derive voltage pulses of decreasing amplitude, means for damping said voltage pulses, means for amplifying the first positive peak of said voltage pulses, a trigger circuit, means for applying said amplified voltage pulses to said trigger circuit, means for damping the input circuit of said trigger circuit, and means for deriving from said trigger circuit a signal of am plitude substantially proportional to the average frequency of: said signals.

2. A device for shaping the waveform of applied signal voltages including, in combination, a saturation amplifier, means for applyingsaid signals to said amplifier to derive a current of substantially square wave form, a tuned circuit, means for shockexciting said circuit by said square wave current to derive voltage pulses of decreasing amplitude, means for amplifying the first positive peak of said voltage pulses, a trigger circuit, means for applying said amplified voltage pulses to said trigger circuit, and means for deriving from said trigger circuit a signal of am plitude substantially proportional to the average frequency of said signals.

3. A device for controlling the waveform of an output signal in response to an applied signal voltage including, in combination, a saturation amplifier, means for applying said signals to said amplifier to derive a current of substantially square wave form, a tuned circuit, means for shock exciting said circuit by said square wave current to derive voltage pulses of decreasing amplitude, means including a diode for damping said voltage pulses, means for amplifying the first positive peak of said voltage pulses, a trigger circuit, means for applying said amplified voltage pulses to said trigger circuit, means including REFERENCES CITED The following references are of record in the me of this patent:

UNITED STATES PATENTS Number Name Date 2,153,202 Nichols Apr. 4, 1939 2,159,493 Wright May 23, 1939 2,266,668 Tubbs Dec. 16, 1941 2,277,000 Bingley Mar. 17, 1942 2,307,316 Wolff Jan. 5, 1943 2,408,061 Grieg Sept. 24, 1946 

